Long-distance telephone charges of IP FAX can be reduced by comparing with that of FAX based on the conventional public switched telephone network. With an application of next generation network (NGN), IP FAX will be popularized quickly. The process for switching voice signal to fax signal of the conventional technology of IP FAX is impossible to avoid. It is no doubt that the normal process of fax can be affected if the process for switching signal can not be implemented in a manner of synchronization. Therefore, an application of IP FAX lies on whether the synchronous switch can be conducted reasonably and thoroughly.
Certainly, whether events can be transmitted smoothly by fax also depends on whether fax machines themselves as to be communicated each other are compatible. However, it is impossible for IP FAX of the prior art to implement a thorough testing for compatibility relation between IP Fax technology and each kind of fax machines, as a variety of fax machines have been already used. It is also unpredictable for fax machines what performance parameters will be developed in future. Meanwhile, the media switch is a necessary process for implementing an application of IP FAX. Therefore, it has been already a key technology how to make both media gateways at transmitting side and receiving side of fax event implement media switch synchronously so as to improve rate of successful transmission by fax.
Methods for implementing media switch for IP FAX are already disclosed. As a kind of methods of the prior art, whether a fax event can be detected at time division multiplex side (TDM) is considered to be the unique condition for switching. However, it is possible for the media gateway at transmitting side or at receiving side to be unable to detect a fax event, for the fax machine at transmitting side could not active V.21 signal (based on the 300 bps fax operation standard, ITU-T V.21) before receiving a whole V.21 signal transmitted from the fax machine at receiving side due to the requirement that fax signals at both sides need to be coupled each other. In case of this situation occurred, failure for media switch will be resulted in.
Another method is disclosed in the prior art that such a media switch is controlled by a media gateway controller. According to this kind of method, signaling is simply guaranteed to be sent at the same time from the MGC. Obviously, the media switches implemented by different gateways could not be synchronous once the routes between the MGC and the media gateway at transmitting side and between the MGC and the media gateway at receiving side are different, as the signaling could not reach each gateway at the same time. Therefore, the method for media switch of the prior art could result in failure of transmission for fax event due to time difference.